Britbike forum Forums British Motorcycle specific Brands Norton bulletin board 1972 Commando Oil Scavenge Issue

There's a good description of the problem and the fix on the Andover Norton website Here.

It seems that the original problem was related to riders losing all the oil as it blew out of the breather at high RPM. This was noticed mainly in Germany where riders could open their bikes up on the Autobahn and also by riders on racetracks.

Your problem of oil filling the crankcases is quite different and may be related to an issue with the scavenge side or oil pump and lines.

As well as checking the oil pickup in the cases, check the pump return gears and how it mounts on the cases. Also, check the condition of the rubber return lines. Sometimes a flap of rubber forms where the rubber pipe is pushed onto the metal pipe.

Wasn't the simple 'fix' for this to plumb the breather back into the oil tank return. ?

This meant that most - if not all - the scavenging was via the breather ....
Not ideal.
But if it works, then hey - why not.



Aye ...
A sump full of oil usually shows up though as oil coming out everywhere, or the engine getting all hot and bothered ?

Before I rebuilt my combat engine, I could see oil returning to the tank via the breather, as far as I know, this only happened on startup but it's possible that it happened whilst riding. This happened mainly because the bike was a frequent wet sumper, which was cured by fitting a new pump and a MK3 timing cover which has the anti-wet sump valve, which works.

My breather is plugged into the oil tank so any oil from the breather simply ends up back in the tank.

As mentioned before, it might be worth reverting to the original breather behind the engine and connecting the breather pipe to the oil tank as per the original.

However, since you already have the breather holes in the crankcase, the next step would be to modify the oil pickup location as shown in the Andover Norton link.

The apparent "foaming" and oil blown from the oil tank breather was caused by the scavenge problem so they are not unrelated.


https://andover-norton.co.uk/en/sicombatcrankcase/
"This new crankcase, and its breather arrangement, had been so cleverly redesigned for 1972 the sump was not drained properly at higher revs. Normally the problem starts from 4.000 to 4.500rpm upwards. As a result oil foam formed in the crankcase that was then thrown out of the breather, mostly unnoticed by the rider sitting in front of the breather outlet, until there was no oil left."

Agreed, and what's puzzling is how the contents of the oil tank were lost by excess oil from the breather.

I guess the excess oil blowing from the engine breather somehow caused oil to blow out of the oil tank breather until eventually it was all gone. Must have been riding at quite some speed for this to happen!

It would be the foam blown from the crankcase engine breather to the oil tank and then the foam would have been blown from the oil tank breather.

With years of discussion, I am not convinced anyone "knows" or is telling what is really happening inside the engines and oil tank and venting systems.
The evolution can be seen on the display tables behind me with 25 NHT display of all variants from series 12 through 325xxx. for the 20, 20M3 then 20M3S had no weir. then after a long 100mph+ run I blew up a barely broken in 70 20m3S roadster...In 71 a new bigger oil tank (roadster) replaced the central tank why? So evolution to the 200000/combat breather with new weir and blowing oil out the rear exit, may(?) have helped except for eating trash and killing the oil pump.

Finally the 300000 series had the weir and screened sump pickup and timing chest vent. Was this the NHT final best engine? remember the 325 series lost the weir

A man has to know his limits and that may be true for the NHT as well.

When the froth tower appeared on the dommie oil tanks, wasn't it said at the time that they were 10 times too small ?

Maybe multigrade is more frothy than monograde ???? !!!!



(I'll see if I can get/find a better pic)

We diverge, a tad.

No author is declared of the AN 72-73 mod article, which seem to be a morphed combination of INOA/Old Brit of the early 90's plus a more recent change to allow drain out and use of the timing chest as an expansion chamber. Therefore the crankshaft area is still the oil scavenge source and oil is not any better defrothed.
Unfortunately, as a contributing editor of the INOA tech digest V3.2, I was restricted as to the areas I was given/allowed to contribute and comment on.
Since then the engine and oil control section in my copy is now filled with "red ink" notes". Also, FWIW, the INOA tech digest project is dead.

Has anyone put clear tubing in the combat breather return to view the return oils condition? flow volume and froth? Please speak up!
Other NHT variants 12-122--14-18-20-20M3-20M3S-300000-325?-hybrids?

Interesting suggestion.

All dry sump systems must flow more air than oil, sometimes.
Its a function of having enough capacity to (sometimes) deal with lotsa oil in the sump ?

Whether they all froth it to the same extent may be another matter ...

P.S. Edit.
I'd comment that my first Model 7 dommie came with plastic tube oil lines.
'Clear' is a debatable term here, they were somewhat discoloured by then.
But bubbles were viewable at idle, on the stand. Quite a lot of air even ?
Examining them on the fly may be a life endangering exercise !

PROBLEM SOLVED!!!

Okay, so I have now split the crankcases and it all looks good. Or rather, it did until I shone torch into the scavenge oil feed passage on the right hand crankcase.

Inside the 1/4in diameter passage, just at the point where the drilling from the crankcase oil scavenge inlet meets the drilling from the crankcase joint and where the passage effectively turns 90 degrees, I could make out a piece of shiny metal which was obviously obstructing the flow. With some careful work with bits of bent wire, I managed to pull out the little bugger.

It turned out to be a piece of mis-shapen aluminium alloy measuring approx 9mm long by 5.6mm by 3.3mm. Probably a piece of piston from some previous mishap. It wouldn't even have been visible had it been stuck further back in the oil passage. In fact it must have been in there at the time when this engine was rebuilt. At the time, a blow-through with an airline would have shown that the oil passage wasn't blocked, or rather, not completely blocked, but it wouldn't have shifted the obstruction either.

What is shocking is how a piece of metal of that size could find its way into a 6.4mm dia oil passage. Even more shocking is that Norton's engineers designed the scavenge system without any form of strainer to prevent such a thing happening.

So, my next task is to block up the original 1/4in oil intake either with a suitable epoxy compound or with a set screw, and then to create a new scavenge intake at the rear of the casing as described by Dave Comeau in his link earlier in this thread. By employing several 3mm drillings, the same thing will never happen again. And even in the unlikely event that one of the scavenge intake holes became blocked with debris, there would be two or three other holes remaining to provide a continuous flow of oil back to the pump.

It is very fortunate that I was able to spot this issue early and before it could do any damage.

At one point whilst attempting to analyse the situation, I was convinced that the engine breather, having been moved to the timing case, may have been a major factor to the scavenge problem on my engine (the original breather position at the bottom of the crankcase on Combat casings allows it to blow oil as well as air back to the oil tank). But then I realised that with several litres of oil in the crankcase, the scavenge feed should still be well below the level of oil accumulated in the crankcases and therefore would still have fed the oil pump, unless there was so much cavitation that mainly air was getting sucked into the oil pump. This seemed unlikely bearing in mind the engine wasn't being worked hard as it was being gently run in, so I felt that there had to be another explanation.

And that explanation was a big fat lump of metal obstructing the oil scavenge passage.

Many thanks to all those who have offered their advice and knowledge. What a great group of people on this forum!

I added a screen filter to my 72 cases so the pump is protected.

https://www.accessnorton.com/NortonCommando/72-crankcase-mod-to-add-sump-filter.31727/

I like that solution. True engineering elegance.